1. Field of the Invention
The present invention is directed to forming an optical waveguide grating having a refractive index variation, more particularly to an apodized fiber grating, by varying a fringe contrast of an interference pattern used to expose the waveguide along the waveguide.
2. Description of Related Art
A fiber grating is a modulation of the refractive index of a fiber. An interference pattern of ultraviolet (UV) radiation forms this modulation by locally increasing the index of the fiber core. This interference pattern, and thus the resulting modulation, generally has both a DC and an AC component. The AC component has a submicron periodicity and causes the grating to reflect light at a particular wavelength. The DC component is generally unwanted and causes the grating to be xe2x80x9cchirpedxe2x80x9d, i.e., the grating wavelength varies along the length of the grating.
For wavelength division multiplexing (WDM) applications, fiber gratings are ideally xe2x80x9cpurely apodizedxe2x80x9d, i.e., the AC component has a smoothly varying envelope, e.g., a Gaussian envelope, and the DC component is uniform along the whole length of the grating. FIG. 1A illustrates the index modulation of a grating having an AC component with a Gaussian envelope and a DC component which is also a Gaussian function. FIG. 1B illustrates the index modulation of a grating having an AC component with a Gaussian envelope and a DC component which is constant.
The Gaussian variation or chirp of the DC component of the grating with the index modulation shown in FIG. 1A results in a grating reflection spectrum, shown in FIG. 2A, which is strongly asymmetric, with sidelobes on the short wavelength side. In contrast, the grating having the uniform DC component shown in FIG. 1B has a symmetric reflection spectrum and very low sidelobes. This symmetric wavelength response results in improved filtering performance.
Several methods have been proposed to produce purely apodized gratings. The simplest method of apodizing a grating involves spatially varying the intensity of a beam being used to write to a fiber. This is typically achieved using an interferometer or a phase mask, as discussed below. One such method of spatially varying a beam intensity to form an apodized grating is a double exposure method, in which a fiber is exposed twice to raise the average index of the fiber. This double exposure may be performed either sequentially, as set forth in U.S. Pat. No. 5,309,260 to Mizrahi et al., entitled xe2x80x9cMethod for Forming Distributed Bragg Reflectors in Optical Mediaxe2x80x9d or simultaneously, as set forth in H. Singh and M. Zippin, xe2x80x9cApodized Fiber Gratings for DWDM Applications using Uniform Phase Maskxe2x80x9d, European Conference on Optical Communications Proceedings, 1998. In either scenario, the second exposure does not interfere with the first exposure, i.e., produce fringes, but merely alters the refractive index profile.
Another method for making a purely apodized grating is the use of an apodized phase mask to create the grating, as discussed, for example, in L. E. Erickson et al. xe2x80x9cFabrication of a Variable Diffraction Efficiency Phase Mask by Multiple Dose Ion Implantation,xe2x80x9d J. of Vac. Sci. Tech. B 13(6), pp.2940-3, November 1995. This results in a constant average index of refraction along the grating, while the index modulation at the ends of the grating approaches zero. This apodized phase mask approach lacks flexibility, since a different phase mask is required for every different grating wavelength or apodization profile.
Yet another method for making a purely apodized grating involves moving the phase mask and the fiber relative to one another during scanning by a beam. In this techniques, apodization is achieved by dithering the relative phase between the phase mask and the fiber at the edges of the grating, as set forth, for example in R. Kashyap et al., Electronics Letters, Vol. 32 (15), pp. 1394-6, 1996. While this technique alone is limited to the length of available phase masks, longer gratings can be made by scanning several such masks, while trimming the discontinuities between sections, as disclosed in M. I. Cole et al., Electronics Letters, Vol. 31 (17), pp. 1488-9, 1995. Alternatively, longer gratings may be created by significantly overlapping the footprint of the writing light beam with previous lines to average the writing process, as set forth in PCT Application Number PCT/GB97/02099 to Laming et al., entitled xe2x80x9cFabricating Optical Waveguide Gratingsxe2x80x9d published on Feb. 26, 1998. While this technique is flexible, it is difficult to implement and requires expensive, accurate equipment.
The present invention is therefore directed to a method of creating apodized gratings which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
The above and other objects may be realized by exposing a fiber to ultraviolet (UV) interference patterns with a uniform average intensity, but with a varying fringe contrast.
At least one of the above and other objects may be realized by a method of fabricating an optical waveguide grating having a refractive index variation including providing a photosensitive optical waveguide, splitting input light into two beams, supplying the two beams to the photosensitive optical waveguide, exposing the photosensitive optical waveguide to an interference pattern formed by the supplying of the two beams, and varying a fringe contrast of the interference pattern along the photosensitive optical waveguide.
The varying may include altering a relative polarization between beams used to generate the interference pattern. The altering may include rotating a polarization of one of the beams used to generate the interference pattern.
The varying may include altering relative intensities of beams used to generate the interference pattern. The altering may include providing a mask having openings with sizes varying with position for each of the beams. The providing a mask may include creating a pair of masks includes generating a pair of masks designed to provide a substantially constant total intensity on the optical waveguide. The exposing may include providing a beam of uniform intensity to each of the masks.
The varying may include rotating a polarization of an input beam and using a polarizing beam splitter to split the input beam into beams used to generate the interference pattern. The varying may further include controlling said rotating such that the relative intensities of the beams used to generate the interference patter are related as follows:
I1(z)=xc2xd(1xc2x1{square root over (1xe2x88x92C(z)2)})
I2(z)=xc2xd(1∓{square root over (1xe2x88x92C(z)2)})
where z is a position along the waveguide, I (z) is the intensity of a beam used to generate the interference pattern at a position z, and C(z) is a desired contrast function.
The varying includes rotating a polarization of an input beam, deflecting a portion of the input beam having a first polarization from a portion of the input beam having a second polarization orthogonal to the first polarization, and amplitude splitting the input beam into the beams used to generate the interference pattern. The varying may further include controlling the rotating such that the relative intensities of the beams used to generate the interference patter are related as follows:
I1(z)=xc2xd(1xc2x1{square root over (1xe2x88x92C(z)2)})
I2(z)=xc2xd(1∓{square root over (1xe2x88x92C(z)2)})
where z is a position along the waveguide, I (z) is the intensity of a beam used to generate the interference pattern at a position z, and C(z) is a desired contrast function.
The exposing may include scanning a beam along the photosensitive optical waveguide. The varying may include altering a relative polarization between beams used to generate the interference pattern simultaneously with the scanning to generate a desired grating. The varying may include altering relative intensities of beams used to generate the interference pattern in accordance with the scanning to generate a desired grating.
The above and other objects may be realized by providing an optical waveguide grating made by any of the above methods.
The above and other objects may further be realized by providing an apparatus for fabricating an optical waveguide grating including a splitter which splits input light into two beams to be delivered to an optical waveguide to form an interference pattern thereon and a fringe contrast varying system which alters a fringe contrast of the interference pattern along the optical waveguide to create a desired contrast function.
The fringe contrast varying system may include a scanner which scans the input light along the optical waveguide, a rotatable polarizer which alters a polarization of one of the two beams relative to the other, and a controller which adjusts a polarization presented by the rotatable polarizer in accordance with the position of the input light and the desired contrast function.
The fringe contrast varying system may include a scanner which scans the input beam along the optical waveguide, a rotatable polarizer which alters a polarization of the input beam, and a controller which adjusts a polarization presented by the rotatable polarizer in accordance with the position of the input light and the desired contrast function, wherein the splitter is a polarizing beam splitter.
When the splitter splits the input light into two beams of substantially equal amplitude, the fringe contrast varying system may include a scanner which scans the input beam along the optical waveguide, a deflector which alters the optical path traversed by a portion of the input beam having a first polarization from a portion of the input beam having a second polarization, orthogonal to the first polarization, a rotatable polarizer which alters a polarization of the input beam, and a controller which adjusts a polarization presented by the rotatable polarizer in accordance with the position of the input light and the desired contrast function.
The fringe contrast varying system may include a pair of masks, one mask for each of the two beams, the masks providing a varying opening size along the optical waveguide, a total intensity output by the pair of masks being substantially constant. The fringe contrast varying system may further include a scanner which scans the input light along the optical waveguide. The input light may have a substantially uniform intensity along the optical waveguide.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.